1. Field of the Invention
This invention relates to timing the registry of meshing teeth in a polymer pump that contains polymer.
2. Description of the Prior Art
Pumping apparatus that pumps molten polymer (polymer) and pressurizes that polymer can contain a pair of opposed shafts, each shaft carrying teeth that force viscous polymer from the inlet of the pump to its outlet. The pressure under which the polymer exists at the outlet of the pump is substantially elevated above the pressure existing at the inlet of the pump. For example, with high density polyethylene (HDPE), the inlet pressure can be from about 30 to about 40 psig at from about 3500 to about 5500 Fahrenheit (F), whereas the outlet pressure can be from about 2,000 to about 3,000 psig at from about 375° to about 575° F.
The polymer fills the space between the teeth at the inlet side and is conveyed to the outlet side of the pump, after which the teeth are brought to their point of closest approach, i.e., cyclically into meshing engagement with one another, the engagement serving to exclude the polymer and generate pressure. The design of the teeth is such that the clearance between adjacent surfaces is minimized in part to prevent back flow of polymer from the high pressure outlet side of the pump back into the lower pressure inlet side. The greater this back flow of polymer, the less efficient the operation of the pump, causing the pump's turning speed to be increased to compensate, and wasting energy in the operation of the pump.
Accordingly, to prevent this undesired back flow of polymer, the registry of the pump teeth relative to one another when in meshing engagement must be timed to be very close, but without any actual physical contact of the meshed teeth. If the teeth contact one another when meshed, premature and undesired wear of the teeth occur thereby not only allowing back flow of polymer, but also requiring shutdown of the pump and an expensive, premature reworking of the worn teeth. Since each shaft of such a pump can cost as much as $100,000, it is desirable to maintain the non-touching registry of the teeth on these opposed shafts for as long as possible. For example, when the desired non-touching teeth registry is maintained, the operating life of such a pump can extend for up to 5 years, whereas if touching during pumping occurs, this life span can be reduced to 2 years at the very best.
However, to prevent polymer back flow, the gap (tolerance) between adjacent teeth when in meshing engagement must be quite small, about 0.02 of an inch in the case of HDPE. The opposing teeth bearing shafts are fixed relative to one another to maintain this non-touching timing.
When a pump is new and contains no polymer, the teeth are clean of polymer and the desired non-touching gap registry between adjacent meshed teeth can easily be achieved even in the field, e.g., when installed in the plant. This is so because one can readily obtain access to the interior of the pump and physically gauge the gap between adjacent meshed teeth before the opposing shafts are fixed to one another to maintain this registry while the pump is in operation.
However, from time to time, maintenance of gear boxes, couplings, and the like must be carried out on any pump, and at such times it may be necessary to stop the operation of the pump. This leaves the pump full of polymer, and its teeth covered with polymer. During such maintenance work, it may be necessary to remove the equipment that keeps the shafts and their teeth registry constant thereby causing the loss of the desired non-touching tolerance between adjacent meshed teeth. Since the pump is full of molten polymer, access to the interior of the pump to re-set the timing (registry) of the pump teeth is much more problematic. The polymer could be removed from the interior of the pump and from around the meshed teeth, but this is a time-consuming and costly approach.
It is much more desirable, and cost effective, to be able to re-set the timing of the pump teeth registry from outside the pump without requiring access to the interior of the pump, so that maintenance procedures can be completed. This invention provides such a method.